Validating Next Generation Therapeutics and Targets in Castrate-Resistant Prostate Cancer

Abstract

Aberrant androgen receptor (AR) signalling is a key driver of prostate cancer (PC) manifestation and progression. Therefore, therapeutic intervention has focused on interrupting the AR signalling axis using androgen deprivation strategies and direct receptor antagonists, such as enzalutamide, collectively termed hormone therapy. Despite initial efficacy, hormone therapy ultimately fails due to a cohort of resistance mechanisms which enable reactivation of AR signalling and PC progression to a more aggressive form called castrate resistant prostate cancer (CRPC). Clinically, CRPC presents as a disease with minimal effective treatment options and therefore, new targets and therapeutics are required to improve patient outcome. Of the characterised mechanisms of hormone therapy resistance that enable progression to CRPC point mutations within the ligand binding domain of the AR enhance promiscuity of the receptor to promote binding and activation by alternative ligands, including enzalutamide. Crucially, next generation compounds are currently in development to improve targeting of clinically-relevant mutated forms of the AR. Other resistance mechanisms include androgen bypass in which AR is controlled by other signalling pathways, including kinase cascades such as PI3K-AKT. Consistent with this phenomenon the kinase SGK1 has been linked to AR signalling and has been postulated to be a potential therapeutic target in CRPC. The aims of this study were to: • better understand the mechanism of action and efficacy of the newly developed JNJ-Pan-AR compound, which is a non-clinically-relevant derivative of JNJ-63576253, across a cohort of CRPC relevant cell lines • validate SGK1 as a therapeutic target and understand whether SGK1 regulates AR activity. This study showed the JNJ-Pan-AR has antagonistic activity comparable to enzalutamide in CRPC cell lines expressing wild-type AR, and significantly outperforms enzalutamide in a clinically-relevant ARF877L point 10 mutant-expressing cell line. JNJ-Pan-AR mediates these effects by binding AR and preventing nuclear translocation and enrichment at key cis-regulatory elements of AR-target genes. For the second aim of the project, it was concluded that SGK1 knockdown significantly reduces proliferation of a cohort of CRPC cell lines, and significantly effects a subset of AR target genes. However, these results were not emulated using SGK1 inhibitors previously used in the literature (GSK650394), with publicly available kinase screen data suggesting that these compounds are highly un-selective. Gene set enrichment analysis (GSEA) on RNASeq data suggested that SGK1 drives a cohort of genes with a similarity to neuroendocrine prostate cancer (NEPC) associated pathways MYC and mTOR. In all, the work has helped define the mechanism of action of a novel next generation anti-androgen which will be of value for understanding which patients would benefit from the clinical candidate compound JNJ-63576253. Furthermore, although the evidence suggests that SGK1 is not strongly implicated in AR regulation, its depletion rather than kinase inactivation using a selective inhibitor impacts prostate cell proliferation suggesting a potential scaffolding role of SGK1 in controlling cell fate. Future studies are required to help define this phenomenon.